Known pressure measuring devices ascertain pressure of a measured medium in a container either by means of a capacitive or a piezoresistive pressure measuring cell with a correspondingly designed measuring bridge, or with a correspondingly designed measuring element, either of which is usually an integral part of a chip. In both embodiments, the measurement signal representing pressure is also dependent on temperature. In order to eliminate the temperature influence on the measured value and so to reach a desired high accuracy of measurement, it is necessary to compensate the disturbing variable ‘temperature’ in a suitable manner.
From the state of the art, known solutions propose to compensate the disturbance ‘temperature’ either analogy via a corresponding compensation circuit or digitally via a characteristic curve examination in the case of different selected temperature levels in the working range of the pressure measuring device. An analog compensation circuit is disclosed, for example, in German Patent DE 35 03 489 A1. The digital compensation uses an algorithm, which enables removal of a measurement error arising due to a temperature change by calculation at the pressure measuring device. A known algorithm is based, for example, on the method of least squares, or linear regression.
Applied is also the following algorithm: On the basis of comparison of measured values, provided via the measuring bridge, with corresponding reference values, via a mathematical model with an equation system of nth order, coefficients for minimizing the characteristic curve error and temperature error are ascertained. The coefficients are then stored in a sensor electronics assigned to the pressure measuring cell, or the pressure sensor. Alternatively, for this, also a grid of known points can be stored, wherein, between these known points, an interpolation is performed. The coefficients are likewise stored in a memory of sensor electronics.
The earlier described methods for compensating the temperature effect in the case of pressure measuring devices is not without problem in all applications: Thus e.g. the characteristic curve examination for ascertaining the temperature dependent correction values occurs always at a point in time, when the pressure measuring cell is ‘warmed through’, when, thus, all mechanical components of the pressure measuring cell, especially the housing platform, the measuring flanges and the pressure transfer medium, after an abrupt temperature change, are in an ‘equilibrated’ steady state and, thus, at the same constant temperature.
These methods cannot handle all measuring situations arising in practice, an example being the case of a pressure measuring device, which ascertains temperature, for example, at the measuring element, which—as already mentioned above—is, in many cases, an integral part of a chip, which is in contact with the measuring membrane. In many cases, the measuring membrane is isolated from the measured medium by at least one pressure transfer means. Known compensation models compensate the temperature influence nevertheless on the basis of the temperature registered at the measuring element. At times, this can cause a relatively large measurement error.
The problem, which can result from this situation, is made clear by the following example: If the surrounding temperature changes abruptly, then the temperature of the pressure measuring cell changes internally significantly slower than the currently measured temperature at the measuring element. This shows itself especially when the chip with the integrated temperature sensor is arranged outside the actual housing of the pressure measuring cell, or the solid platform, such as is the case for the pressure measuring cell schematically illustrated in FIG. 2.
If the surrounding temperature changes, now, for example, from 20° C. to 70° C., then the temperature sensor at the measuring element, or on the chip, displays relatively rapidly the 70° C., which reigns in the surroundings. In contrast, such temperature reaches the mechanical components of the pressure measuring cell only after a significantly longer time, namely when the pressure measuring cell has ‘warmed through’. If the known compensation method is employed, then, over a period of time of approximately two hours, the temperature compensation will be performed on the basis of a temperature value, which is too high. As a result thereof, the pressure measuring device delivers during this time span a pressure measurement value burdened with a relatively large measurement error.
Other problematic cases of application occurring in practice are set forth, by way of example, as follows:    Case 1: At a relative, absolute or difference, pressure measuring device, a change, especially an abrupt change, occurs in the surrounding temperature.
In this case, primarily the temperature of the measurement transmitter, which contains the electronic components of the pressure measuring device, changes. Examples of this case are temperature changes as a result of change of solar radiation or as a result of movement of a missile. Also installation of the pressure measuring device in the vicinity of the measured medium has a large influence, moreover, when the device is temporarily exposed e.g. to the radiative heat from open, firing hatches or during metal tapping in foundries.    Case 2: At a difference pressure measuring device, the temperature, in the case of an abrupt change of temperature of the measured medium, changes in both pressure measuring chambers almost simultaneously. Examples of this case are fill-level measurements at containers with variable temperature of the measured medium, e.g. the filling of a hot liquid into a container residing earlier at room temperature. This situation can occur, in among other ways, in the context of cleaning cycles or in the case of flow measurements with orifices where the measured media have a variable temperature.    Case 3: At a relative, absolute or difference, pressure measuring device, the temperature of the measured medium changes unilaterally. An example of this is fill level measurement in a container having a unilateral flange mount—here, thus, the reference side is open. This situation can occur, for example, in the case of pressure measurement in an open container.